The Collaborative International Dictionary of English v.0.48:

Imam \I*mam"\, Iman \I*man"\, Imaum \I*maum"\, n. [Ar. im[=a]m.]
   1. Among the Mohammedans, a minister or priest who performs
      the regular service of the mosque.
      [1913 Webster]

   2. A Mohammedan prince who, as a successor of Mohammed,
      unites in his person supreme spiritual and temporal power.
      [1913 Webster]

WordNet (r) 3.0 (2006):

imam
    n 1: (Islam) the man who leads prayers in a mosque; for Shiites
         an imam is a recognized authority on Islamic theology and
         law and a spiritual guide [syn: imam, imaum]